1. Technical Field
The present invention relates to a method for identifying image, and more particularly, to an illumination compensation method with high image recognition accuracy.
2. Description of Related Art
In recent years, digital video signal processing is very popular because digital audio and video technology have made a lot of progress, the price of large data storage is lower and the cost of the optical photographic equipments also decreases. Most importantly, artificial intelligence and computer vision technology are getting mature. So intelligent video processing systems gain much attention to the public, especially it has become a very important role in the safety monitoring field. In this field, the accuracy of face recognition is an essential goal to pursue, so the present invention addresses this issue here, and provides a high accuracy of face recognition.
For face recognition, there are several problems which will affect the recognition accuracy. Among them, ambient lighting variation is a very crucial problem because it will affect the system performance considerably. Currently, most face recognition methods of the prior arts assume that human face images are taken under uniform illumination, but in fact the background illumination is usually non-uniform and even unstable. Therefore, the face images of the same person often have very different appearances which make face recognition very difficult. Furthermore, slanted illumination probably produces different shadows on face images which may reduce the recognition rate greatly. So the present invention focuses on this topic and provides an illumination compensation method to improve the recognition accuracy under different background illumination.
U.S. Pat. No. 6,122,408 discloses a method for normalizing the lighting changes in an input image having various illuminating intensities. The method filters the input image through a low-pass filter to provide a background estimation, and then employs an adaptive local histogram modification with background estimation as a preprocessing step of the input image. However, the recognition accuracy of this method is only 76.6%, which is not accurate enough for face recognition.
Moreover, U.S. Pat. No. 6,445,812 discloses an illumination compensation system for correcting smooth intensity variations due to illumination changes is based on an assumption that an underlining image reflectance function is approximately a piecewise constant and that an image irradiance function is spatially smooth. The system first takes the logarithm of an image brightness function. Gradient constraints are then computed using a finite difference. Nevertheless, the face images are not smooth images, thus making the face images of the same person often have very different appearances. Accordingly, the bias of the evaluation of the recognition rates in this method would be remarkably enhanced. In addition, the performance of the evaluation of the recognition rates of this method are not provided and thus the recognition accuracy of this method remains unknown.
Furthermore, U.S. Pat. No. 7,020,345 discloses a method for illuminant compensation of an image of a human face, wherein the image includes a plurality of color pixels. This method adopts the assumption recited in U.S. Pat. No. 6,445,812, nevertheless, the face images are not smooth images, thus making the face images of the same person often have very different appearances. Therefore, the recognition accuracy of this method is only 81.7%.